Emulsified, low pH cosmetic compositions have been sold to consumers for the purpose of providing smoother, younger-looking skin. The emulsified cosmetic compositions often contain an organic acid, such as a hydroxycarboxylic acid, hereafter termed a "hydroxy acid", and typically contain an .alpha.-hydroxycarboxylic acid, hereafter termed an ".alpha.-hydroxy acid". Other types of organic acids are included in cosmetic compositions as emollients.
Emulsified cosmetic compositions containing a hydroxy acid act to smooth fine lines in the skin and lighten skin dark spots caused by photoaging, i.e., skin damage resulting from excessive exposure to the sun. The emulsified, low pH cosmetic compositions originally were available only by prescription, but over the last several years several over-the-counter compositions have become available to consumers. The over-the-counter compositions generally include low weight percentages of a hydroxy acid, e.g., up to about 8% by weight. The over-the-counter hydroxy acid compositions make skin smoother and help skin retain moisture better.
The acid present in cosmetic compositions often is a hydroxycarboxylic acid, generally an .alpha.-hydroxycarboxylic acid, like glycolic acid. However, some low pH cosmetic compositions incorporate acids other than hydroxy acids. The .alpha.-hydroxy acids have been used by dermatologists for over twenty-five years to treat eczema, psoriasis and ichthyosis, an inherited skin condition. .alpha.-Hydroxy acids also effectively treat dry, flaking skin on the hands caused by excessive exposure to detergents.
On normal skin, a hydroxy acid works as an exfoliant to facilitate shedding of dead skin cells from the skin surface and facilitate replacement of the dead cells with new skin cells at a faster rate. Exfoliation is a normal skin regeneration process, but aging and exposure to the sun slows exfoliation. Individuals therefore lose the ability to efficiently shed dead skin cells, and as a result the skin gets flaky. The cells therefore shed in clumps, and the skin is perceived as dry. A hydroxy acid stimulates skin cells to shed faster and more smoothly. A hydroxy acid therefore normalizes the scaling process and weakens the bonds between scales in photoaged skin. By normalizing the skin's outer layer, a hydroxy acid also improves the skin to ability to hold moisture.
In particular, dermatologists apply compositions having a high concentration of hydroxy acid (50 to 70 weight percent) for superficial peels, to smooth rough skin, and to remove fine lines, acne scars, age spots, irregular pigmentation, and precancerous scaly patches. Dermatologists and salon professionals use compositions having a moderate hydroxy acid concentration (10 to 50 percent) to help control acne by unplugging pores, and to enhance the effectiveness of Retin-A and skin bleaches. At these concentrations, the hydroxy acid-containing products often provide dramatic results, but the potential to irritate or burn the skin is high. At hydroxy acid concentrations of 30% by weight or more, the compositions are capable of chemically burning the skin.
A low pH cosmetic composition generally is applied after cleansing and toning the skin, and before moisturizing. Some acid-containing compositions also contain a moisturizer, which eliminates the final moisturizing step. With regular use, the hydroxy acid-containing compositions essentially eliminate the need to use a mechanical exfoliator, such as a scrub.
Compositions containing a hydroxy acid or other organic acid are acidic in nature and have a low pH. Accordingly, there is a need to balance the acidic nature of an emulsified, low pH cosmetic composition with the irritation potential of the composition because acid-containing compositions have a strong skin irritation potential. Many acid-containing compositions, including hydroxy acid-containing compositions, warn the user that a tingling or burning sensation may be felt after the first several applications of the composition to the skin. For esthetics and consumer confidence it is important to minimize the tingling or burning sensation. In addition, for prescription compositions and compositions used in doctor's offices, it is advantageous to reduce the possibility of a chemical burn, or the irritation caused by a more aggressive acid treatment, such as a skin peel or an acne scar removal.
One problem encountered in formulating emulsified, low pH cosmetic products containing an acid is pH drift. During storage, an emulsified, low pH composition typically exhibits a downward pH drift of at least about 0.2, and usually about 0.3, pH units. This downward drift in pH increases the acidity of the composition, and accordingly increases the skin irritation potential of composition. The pH drift also can lead to emulsion instability thereby causing phase separation during storage.
Phase separation is an important problem to overcome because the cosmetic compositions typically are emulsified creams or lotions. Phase separation can lead to an acid concentration in the water phase which either reduces product efficacy if the predominantly oil phase is applied to the skin, or increases skin irritation potential if the predominantly water phase is applied to the skin. The present invention is directed to overcoming these problems and providing pH-stabilized and phase-stabilized, emulsified cosmetic compositions containing an acid, and particularly a hydroxy acid.